Mesozoic of SPITI

Three major cycles of deposition have been found in the sedimentary records of the Mesozoic formations of the Spiti Basin.

1. The Triassic succession consists of predominantly carbonate rocks that were deposited in a shelf sea.
2. The second phase phase of deposition is represented by the Middle Jurassic to Lower Cretaceous sequence of predominantly argillaceous facies.
3. The third and final cycle is represented by Upper Cretaceous sequences that were laid down in shallow to deep-water marine basins under fluctuating tectonic conditions.

The term Lilang System used for the Triassic rocks lie above the Productus shale of Late permian age and below Kioto limestone.

• The Lower Triassic (Bunter) that has is overlain by thick Middle Triassic (Muschelkalk) and thick Upper Triassic (Keuper) sequence.

• A phase of marine regression and possible sub-aerial erosion is marked by the presence of a limonitic pebble layer at the contact of the Lower Triassic and underlying Productus shale. The new phase of marine transgression began with the deposition of the Otoceras beds.

Upper Triassic (Bunter) :- Each of the unit in this is characterized by the presence of zone fossils belonging to cephalopod mollusca.

Middle Triassic (Muschelkalk) :- The succession is richly fossiliferous and contains the characteristic fossils of brachiopods, cephalopods and bivalves.

Upper Triassic (Keuper) :- It is the thickest of all the Himalayan Triassic subdivisions. The succession contains some fossiliferous beds, which yielded species of brachiopods, bivalves, crinoids and corals.

This Lilang system is overlain by the Kioto limestone/ Megalodon limestone. It is divided into two parts as, Para Member and Tagling Member.

All other formation and beds are collectively known as Kibber Group.

“The Kioto -Spiti Shale contact represents a hiatus from Late Callovian to Early Exfordian”

The Spiti Shale demarkates the Jurassic age. The lowermost unit of Spiti shale is Belemnite Shale, middle unit if Chidamu beds and uppermost unit is Lochambal Beds shows a decrease in calcareous and increase in silt content.

The Giumal and Chikkim formation represents the cretaceous time period. The chikim formation represents a shallow water and euxinic (reducing) conditions . The uppermost unit is overlain by the Quaternary fluvial and glacial sediments. This may represent a faster pace of withdrawal of the Tethys paleo-sea at the end of the Mesozoic Era.